1. Field of the Invention
The present invention relates to an optical film, and more particularly to an optical film with a plurality of prism rod structures required for a backlight system of a liquid crystal display or other electronic devices.
2. Description of Related Art
Please refer to FIG. 1. A backlight system of a general liquid crystal display (LCD) comprises a cold cathode fluorescence lamp (CCFL) 11, light guiding plate (LGP) 12, bottom diffuser 13, prism sheet 14, top diffuser 15 and reflector 16.
Please refer to FIG. 2. A conventional prism sheet is constituted by a plurality of triangular prism rods 141 with their longitudinal axes parallel to each other and a substrate 142. The prism rods 141 are coupled to one side of the substrate 142. A height and a width of each prism rod 141 are the same, and the height of a peak of the prism rod 141 is not varied, a course of the peak is a straight line, two inclined faces at two sides of the peak are symmetrical. The conventional prism sheet 14 is provided with a surface structure constituted by a plurality of regular prism rods 141.
Please refer to FIG. 3. There are three optical path modes of the prism sheet 14:
1. Total Reflection (TRI) Recycle:
When a light beam 143 is projected from an inside of a prism approximately within ±4.degree. relative to a virtual perpendicular axis 140 perpendicular to a bottom face of the prism rod 141, it is influenced by total reflection phenomena of inclined faces of the prism rod 141, recycled toward a back light source after two total reflections and diffused, propagated and utilized continuously;
2. Second Refraction Recycle:
After a light beam 144 is refracted by two prism rods 141, it is then recycled to the back light source and diffused, propagated and utilized continuously;
3. Directed Refraction:
A beam 145 is refracted out through an inclined face of the prism rod 141, this optical path mode have light gathering function; only light beams refracted out within the range of a vision angle 80.degree. of the prism rod 141 can be utilized, as shown in FIG. 3.
Chromatic dispersion and moiré fringes will be easily yielded when a upper layer material film such as a upper differ 15 or another prism sheet is stacked on the prism sheet 14. Furthermore, a face of the prism sheet 14 provided with the plurality of prism rods parallel to each other is rather smooth. If there are defects, they are easy to be found and thus, the prism sheet 14 is considered as a flaw product.
There are many prism sheets with a diffuser layer, for example, a prism sheet disclosed by U.S. Pat. No. 5,844,720 has a plurality of triangular prism rods with their longitudinal axes almost parallel to each other on one side and has the other side roughened. The prism sheet is free from interference fringes, safe from scratching and uniform and high in brightness, being thus useful for liquid crystal display. Furthermore, a prism sheet disclosed by Taiwan Patent No. M293442 comprises a substrate; one side of the substrate comprises a light diffusing layer with raised or indented coarse structure, in which the light diffusion layer has diffusivity of 5% to 90%.
The prism sheets with a light diffusing layer mentioned above are mostly manufactured by allowing a manufactured prism sheet to be further fabricated with a coarse face layer on a bottom thereof; it is rather wastes the labor time. Moreover, a face of the prism sheet with the plurality of parallel prism rods is rather smooth, if there are defects thereon, they are easily found and the prism sheet is considered as a flow product.
A diffusing prism sheet disclosed by Taiwan Patent Application No. 95133798 is provided with a long striped-type coarse diffusion face between two adjacent prism rods. Chromatic dispersion and fringes of prism rods can be respectively effectively eliminated and atomized, moiré fringes are reduced and the yield factor of prism sheets can be increased by mixing light diffused from a diffusion face between two adjacent prism rods and light projected from prism rods. Moreover, a substrate, a plurality of long strip-typed prism rods with longitudinal axes parallel to each other and long strip-typed coarse diffusion faces may be formed into one body such that the prism sheet is easily manufactured.
A diffusing prism sheet disclosed by Taiwan Patent Application No. 96108016 is manufactured by allowing rough coarse portions to be respectively disposed on tops of peaks of a plurality of prism rods and/or two sides of each prism rod so as to reduce block, chromatic dispersion and moiré fringes yielded when the diffusing prism sheet is stacked with the other material film.
However, if the substrate, the plurality of long strip-typed prism sheets with longitudinal axes parallel to each other and the long strip-typed coarse diffusing faces want to be formed into one body, it is rather difficult. Generally, the coarse diffusion face is formed by means of abrasion blasting after the prism rods are formed; the two manufacturing processes are needed, it increases the production time and cost.